U.S. patent number 6,266,494 [Application Number 09/669,105] was granted by the patent office on 2001-07-24 for high-altitude compensation for a xerographic development system.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Roger W. Budnik, Scott L. Kauffman, Richard M. Maier, James M. Pacer.
United States Patent |
6,266,494 |
Budnik , et al. |
July 24, 2001 |
High-altitude compensation for a xerographic development system
Abstract
In a xerographic printing apparatus wherein a development field
is maintained between the photoreceptor and a donor member, there
is always a danger of arcing across the field, particularly at high
elevations. An arcing-avoidance system interacts with the print
quality control system of a printing apparatus, to monitor the
biases within the apparatus being demanded at various times by the
control system. If a bias consistent with arcing conditions is
approached, the arcing-avoidance system constrains the control
system to avoid the arcing conditions. The arcing-avoidance system
accepts as an input the elevation of a particular printing
apparatus.
Inventors: |
Budnik; Roger W. (Rochester,
NY), Pacer; James M. (Webster, NY), Kauffman; Scott
L. (Rochester, NY), Maier; Richard M. (Rochester,
NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
24685038 |
Appl.
No.: |
09/669,105 |
Filed: |
September 25, 2000 |
Current U.S.
Class: |
399/55;
399/285 |
Current CPC
Class: |
G03G
15/0266 (20130101); G03G 15/065 (20130101) |
Current International
Class: |
G03G
15/06 (20060101); G03G 15/02 (20060101); G03G
015/08 () |
Field of
Search: |
;399/55,252,266,285,272,281,290 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
60-140271 |
|
Jul 1985 |
|
JP |
|
61-254958 |
|
Nov 1986 |
|
JP |
|
Primary Examiner: Grainger; Quana M.
Attorney, Agent or Firm: Hutter; R.
Claims
What is claimed is:
1. An electrostatographic printing apparatus having a development
system, the development system comprising:
a donor member;
a charge receptor, a development gap being defined between the
donor member and the charge receptor;
means for creating a development field in the development gap,
whereby toner is conveyed from the donor member over the
development gap to the charge receptor by the development
field;
means for monitoring at least a first parameter of the system to
detect an arcing condition within the development gap;
means for creating at least one of a DC bias and an AC bias in the
development field;
means for creating an initial charging voltage on the charge
receptor; and
means for altering a second parameter of the system to avoid the
arcing condition if an arcing condition is detected, wherein the
second parameter is the initial charging voltage on the charge
receptor.
2. The apparatus of claim 1, wherein the monitoring means operates
on a regular basis when the system is operating.
3. The apparatus of claim 1, further comprising means for
communicating that an arcing condition is detected.
4. An electrostatographic printing apparatus having a development
system, the development system comprising:
a donor member;
a charge receptor, a development gap being defined between the
donor member and the charge receptor;
means for creating a development field in the development gap,
whereby toner is conveyed from the donor member over the
development gap to the charge receptor by the development
field;
means for monitoring at least a first parameter of the system to
detect an arcing condition within the development gap;
a primary developer supply, the primary developer supply providing
toner to be conveyed across the development gap;
a secondary developer supply;
means for selectably transferring toner from the secondary
developer supply to the primary developer supply; and
means for altering a second parameter of the system to avoid the
arcing condition if an arcing condition is detected, wherein the
second parameter relates to deciding to transfer toner from the
secondary developer supply to the primary developer supply.
5. The apparatus of claim 4, wherein the monitoring means operates
on a regular basis when the system is operating.
6. The apparatus of claim 4, further comprising means for
communicating that an arcing condition is detected.
7. An electrostatographic printing apparatus having a development
system, the development system comprising:
a donor member;
a charge receptor, a development gap being defined between the
donor member and the charge receptor;
means for creating a development field in the development gap,
whereby toner is conveyed from the donor member over the
development gap to the charge receptor by the development
field;
means for monitoring at least a first parameter of the system to
detect an arcing condition within the development gap; and
means for calculating a field strength of the development field,
based on at least one parameter from a group consisting of an
amplitude of an AC bias in the development field, a magnitude of a
DC bias in the development field, and a number symbolic of a width
of the development gap, the calculating means being adapted to
calculate at least one local field strength in the development gap,
the local field strength being associated with one of a solid image
area on the charge receptor and a background image area on the
charge receptor.
8. The apparatus of claim 7, wherein the monitoring means operates
on a regular basis when the system is operating.
9. The apparatus of claim 7, further comprising means for
communicating that an arcing condition is detected.
Description
FIELD OF THE INVENTION
This invention relates generally to a development system as used in
xerography, and more particularly concerns a "jumping" development
system in which toner is conveyed to an electrostatic latent image
by an AC field.
BACKGROUND OF THE INVENTION
In a typical electrostatographic printing process, such as
xerography, a photoreceptor is charged to a substantially uniform
potential so as to sensitize the surface thereof. The charged
portion of the photoreceptor is exposed to a light image of an
original document being reproduced. Exposure of the charged
photoreceptor selectively dissipates the charges thereon in the
irradiated areas. This records an electrostatic latent image on the
photoreceptor corresponding to the informational areas contained
within the original document. After the electrostatic latent image
is recorded on the photoreceptor, the latent image is developed by
bringing a developer material into contact therewith. Generally,
the developer material comprises toner particles adhering
triboelectrically to carrier granules. The toner particles are
attracted from the carrier granules to the latent image forming a
toner powder image on the photoreceptor. The toner powder image is
then transferred from the photoreceptor to a copy sheet. The toner
particles are heated to permanently affix the powder image to the
copy sheet. After each transfer process, the toner remaining on the
photoconductor is cleaned by a cleaning device.
One specific type of development apparatus currently used in
high-quality xerography is known as a hybrid jumping development
(HJD) system. In the HJD system, a layer of toner is laid down
evenly on the surface of a "donor roll" which is disposed near the
surface of the photoreceptor. Biases placed on the donor roll
create two development fields, or potentials, across the gap
between the donor roll and the photoreceptor. The action of these
fields causes toner particles on the donor roll surface to form a
"toner cloud" in the gap, and the toner in this cloud thus becomes
available to attach to appropriately-charged image areas on the
photoreceptor.
In any xerographic development system in which there is a
substantial potential relative to the photoreceptor, but
particularly when there exists an alternating current field across
a development gap, there is a practical risk of arcing across the
gap. Such arcing will of course have a deleterious effect on the
operation of the printing apparatus, causing at the very least a
print defect and at worst damage to the apparatus. The various
control systems for maintaining print quality in any xerographic
printing apparatus are liable to cause the various potentials
associated with the xerographic process to reach such levels that
arcing is possible. The risk of arcing is particularly increased in
situations where the printing apparatus is installed at high
altitudes, such as in mountainous regions. The relatively low air
pressure in at higher altitudes can lead to Paschen breakdown, that
is, the ionization of air molecules which leads to arcing, at much
lower potentials than would occur at lower altitudes.
The present invention is directed toward a system in which
conditions conducive to arcing are detected, and the control
systems over the xerographic process are, if necessary, modified to
avoid these conditions.
DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 4,610,531 discloses the basic concept of jumping
development with an AC field set up between a donor member and a
photoreceptor.
U.S. Pat. No. 5,402,214 discloses a control system for a
xerographic printing system in which the reflectivity of a test
patch is measured, and the DC bias of a field associated with the
development unit is adjusted accordingly.
U.S. Pat. No. 5,890,042 discloses a hybrid jumping development
system, in which a donor roll is loaded with a layer of toner
particles by a magnetic roll which conveys toner which adheres to
carrier granules.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided, in an
electrostatographic development system wherein toner is conveyed
from a donor member over a development gap to a charge receptor by
an AC development field in the development gap, a method comprising
the step of monitoring at least a first parameter of the system to
detect an arcing condition within the development gap.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic elevational view of a typical
electrophotographic printing machine utilizing the toner
maintenance system therein;
FIG. 2 is a schematic elevational view of the development system
utilizing the invention herein; and
FIG. 3 is a diagram showing the biases of various elements in a
development system.
FIG. 4 is a flowchart illustrating the arcing-control aspect of a
control system for a xerographic printer according to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
For a general understanding of the features of the present
invention, reference is made to the drawings. In the drawings, like
reference numerals have been used throughout to identify identical
elements. FIG. 1 schematically depicts an electrophotographic
printing machine incorporating the features of the present
invention therein. It will become evident from the following
discussion that the development system of the present invention may
be employed in a wide variety of devices and is not specifically
limited in its application to the particular embodiment depicted
herein.
Referring to FIG. 1 of the drawings, an original document is
positioned in a document handler 27 on a raster input scanner (RIS)
indicated generally by reference numeral 28. The RIS contains
document illumination lamps, optics, a mechanical scanning drive
and a photosensor array. The RIS captures the entire original
document and converts it to a series of raster scan lines. This
information is transmitted to an electronic subsystem (ESS) which
controls a raster output scanner (ROS) described below.
FIG. 1 schematically illustrates an electrophotographic printing
machine which generally employs a photoreceptor belt 10.
Preferably, the photoreceptor belt 10 is made from a
photoconductive material, forming a photoconductive surface 12,
coated on a ground layer, which, in turn, is coated on an anti-curl
backing layer. Belt 10 moves in the direction of arrow 13 to
advance successive portions sequentially through the various
processing stations disposed about the path of movement thereof.
Belt 10 is entrained about stripping roll 14, tensioning roll 16
and drive roll 20. As roll 20 rotates, it advances belt 10 in the
direction of arrow 13.
Initially, a portion of the photoconductive surface passes through
charging station A. At charging station A, a corona generating
device, or corotron, indicated generally by the reference numeral
22, charges the photoreceptor 10 to a relatively high,
substantially uniform potential.
At an exposure station B, a controller or electronic subsystem
(ESS), indicated generally by reference numeral 29, receives the
image signals representing the desired output image and processes
these signals to convert them to a continuous tone or grayscale
rendition of the image which is transmitted to a modulated output
generator, for example the raster output scanner (ROS), indicated
generally by reference numeral 30. Preferably, ESS 29 is a
self-contained, dedicated minicomputer. The image signals
transmitted to ESS 29 may originate from a RIS as described above
or from a computer, thereby enabling the electrophotographic
printing machine to serve as a remotely located printer for one or
more computers. Alternatively, the printer may serve as a dedicated
printer for a high-speed computer. The signals from ESS 29,
corresponding to the continuous tone image desired to be reproduced
by the printing machine, are transmitted to ROS 30. ROS 30 includes
a laser with rotating polygon mirror blocks. The ROS will expose
the photoreceptor 10 to record an electrostatic latent image
thereon corresponding to the continuous tone image received from
ESS 29. As an alternative, ROS 30 may employ a linear array of
light emitting diodes (LEDs) arranged to illuminate the charged
portion of photoreceptor 10 on a raster-by-raster basis.
After the electrostatic latent image has been recorded on
photoconductive surface 12, photoreceptor 10 advances the latent
image to a development station, C, where toner, in the form of
liquid or dry particles, is electrostatically attracted to the
latent image using the device of the present invention as further
described below. The latent image attracts toner particles from the
carrier granules forming a toner powder image thereon. As
successive electrostatic latent images are developed, toner
particles are depleted from the developer material. A toner
particle dispenser, indicated generally by the reference numeral
39, on signal from controller 29, dispenses toner particles into
developer housing 40 of developer unit 38 based on signals from a
toner maintenance sensor (not shown).
With continued reference to FIG. 1, after the electrostatic latent
image is developed, the toner powder image present on photoreceptor
10 advances to transfer station D. A print sheet 48 is advanced to
the transfer station, D, by a sheet feeding apparatus, 50.
Preferably, sheet feeding apparatus 50 includes a feed roll 52
contacting the uppermost sheet of stack 54. Feed roll 52 rotates to
advance the uppermost sheet from stack 54 into vertical transport
56. Vertical transport 56 directs the advancing sheet 48 of support
material into registration transport 57 past image transfer station
D to receive an image from photoreceptor 10 in a timed sequence so
that the toner powder image formed thereon contacts the advancing
sheet 48 at transfer station D. Transfer station D includes a
corona generating device 58 which sprays ions onto the back side of
sheet 48. This attracts the toner powder image from photoconductive
surface 12 to sheet 48. After transfer, sheet 48 continues to move
in the direction of arrow 60 by way of belt transport 62 which
advances sheet 48 to fusing station F.
Fusing station F includes a fuser assembly indicated generally by
the reference numeral 70 which permanently affixes the transferred
toner powder image to the copy sheet. Preferably, fuser assembly 70
includes a heated fuser roll 72 and a pressure roll 74 with the
powder image on the copy sheet contacting fuser roll 72.
The sheet then passes through fuser 70 where the image is
permanently fixed or fused to the sheet. After passing through
fuser 70, a gate 80 either allows the sheet to move directly via
output 84 to a finisher or stacker, or deflects the sheet into the
duplex path 100, specifically, first into single sheet inverter 82
here. That is, if the sheet is either a simplex sheet, or a
completed duplex sheet having both side one and side two images
formed thereon, the sheet will be conveyed via gate 80 directly to
output 84. However, if the sheet is being duplexed and is then only
printed with a side one image, the gate 80 will be positioned to
deflect that sheet into the inverter 82 and into the duplex loop
path 100, where that sheet will be inverted and then fed for
recirculation back through transfer station D and fuser 70 for
receiving and permanently fixing the side two image to the backside
of that duplex sheet, before it exits via exit path 84.
After the print sheet is separated from photoconductive surface 12
of photoreceptor 10, the residual toner/developer and paper fiber
particles adhering to photoconductive surface 12 are removed
therefrom at cleaning station E. Cleaning station E includes a
rotatably mounted fibrous brush in contact with photoconductive
surface 12 to disturb and remove paper fibers and a cleaning blade
to remove the nontransferred toner particles. The blade may be
configured in either a wiper or doctor position depending on the
application. Subsequent to cleaning, a discharge lamp (not shown)
floods photoconductive surface 12 with light to dissipate any
residual electrostatic charge remaining thereon prior to the
charging thereof for the next successive imaging cycle.
The various machine functions are regulated by controller 29. The
controller is preferably a programmable microprocessor which
controls all of the machine functions hereinbefore described. The
control of all of the exemplary systems heretofore described may be
accomplished by conventional control switch inputs from the
printing machine consoles selected by the operator.
Turning now to FIG. 2, there is shown development system 38 in
greater detail. More specifically, a hybrid development system is
shown where toner is loaded onto a donor roll from a second roll,
e.g. a magnetic brush roll. The toner is developed onto the
photoreceptor from the donor roll using the hybrid jumping
development system (HJD) described below. As shown thereat,
development system 38 includes a housing 40 defining a chamber for
storing a supply of developer material therein. Donor roll 42 and
magnetic roll 41 are mounted in chamber of housing 40. The donor
roll 42 can be rotated in either the `with` or `against` direction
relative to the direction of motion of the photoreceptor 10.
In FIG. 2, donor roll 42 is shown rotating in the direction of
arrow 168, i.e. the against direction. Similarly, the magnetic roll
41 can be rotated in either the `with` or `against` direction
relative to the direction of motion of donor roll 42. In FIG. 2,
magnetic roll 41 is shown rotating in the direction of arrow 170
i.e. the with direction. Donor roll 42 is preferably made from a
conductive core which may be a metallic material with a
semi-conductive coating such as a phenolic resin or ceramic.
Magnetic roll 41 meters a constant quantity of toner having a
substantially constant charge onto donor roll 42. This ensures that
the donor roll provides a constant amount of toner having a
substantially constant charge as maintained by the present
invention in the development gap. Metering blade 47 is positioned
closely adjacent to magnetic roll 41 to maintain the compressed
pile height of the developer material on magnetic roll 41 at the
desired level. Magnetic roll 41 includes a non-magnetic tubular
member 92 made preferably from aluminum and having the exterior
circumferential surface thereof roughened. An elongated magnet 90
is positioned interiorly of and spaced from the tubular member. The
magnet is mounted stationarily. The tubular member rotates in the
direction of arrow 170 to advance the developer material adhering
thereto into the nip 43 defined by donor roll 42 and magnetic roll
41. Toner particles are attracted from the carrier granules on the
magnetic roll to the donor roll.
Further as shown in FIG. 2, the magnetic roll 41 and the donor roll
42 are respectively biased in order to convey toner particles from
a magnetic roll 41 to donor roll 42, and then across the gap,
indicated as 200, between of the donor roll 42 and it the surface
of photoreceptor 10. With regard to magnetic roll 41, the bias on
the roll is indicated as Vmag, which is a simple DC bias. Donor
roll 42 is, in turn, biased with both a DC bias, indicated as
Vdonor, and a superimposed AC bias, indicated as Vjump. (The
photoreceptor 10 is typically connected to ground, such as through
a backer bar, not shown, in contact therewith.) The AC on the donor
roll 42 ultimately causes the toner layer on the donor roll 42 to
form a "cloud" of toner near the gap between the donor roll 42 and
the photoreceptor 10: in this way, the free toner particles in the
cloud can attach to appropriately-charged image areas on the
photoreceptor 10.
FIG. 3 is a diagram showing the relative biases on magnetic roll 41
and donor roll 42 for a typical practical embodiment of a
xerographic printer. This practical embodiment will further be
discussed with specific reference to the claimed invention, but of
course the basic principles shown and claimed herein will apply to
any applicable machine design. In this embodiment, for normal
operation, the DC bias on the donor roll 42, Vdonor, is -220 VDC.
Riding on this DC bias on the donor roll 42 is an AC square wave
with an amplitude (top to bottom), Vjump, of 2250V: clearly, a
portion of the total bias on donor roll 42 will enter positive
polarity, as shown. (A typical frequency of the square wave is
about 3.25 kHz.) Magnetic roll 41, under normal conditions, is
biased to -113 VDC, shown as Vmag.
With the particular design of a development system such as shown in
FIG. 2, a high risk location for arcing is the gap G between donor
roll 42 and the surface of photoreceptor 10. Clearly, the biases
Vdonor and Vjump on donor roll 42 will directly affect whether
dangerous arcing conditions exist in the gap at any particular
time. The function of densitometer 180, influencing control system
29, which in turn controls, among other parameters, Vdonor and
Vjump, can cause the general control system, designed to optimize
overall print quality, to lead to possible arcing conditions in the
course of operation of the printing machine.
In order to determine whether possible arcing conditions exist in
gap G, the relevant equations for field strength E for both solid
(i.e., printed small areas) and background (undeveloped or white
small areas) portions of an image are as follows: ##EQU1##
Where:
Vjump is the amplitude (top to bottom) of the AC potential on the
donor roll 42; Vdonor is the DC bias on donor roll 42; Vgrid
(explained below) is the potential on the corotron 22, which places
the initial charge on photoreceptor 10; gap is the width of the gap
between the donor roll 42 and photoreceptor 10; Vimg is the local
potential for a small area on the photoreceptor which is intended
to be developed with toner (i.e., a "solid area"); and Vddp ("dark
decay potential") is the local potential for a small area on the
photoreceptor which is intended to remain white in the printed
image (i.e., a "background area"). Vddp can be reasonably estimated
as Vddp=Vgrid+60 (or some other constant determined from real world
voltage measurements of a particular printer design). Similarly,
Vimg can be reasonably estimated from off-line tests of a
particular printer design.
(Graphic representations of some of the above parameters can be
seen in FIG. 3.)
It will be noted, in the above equations, that of the various
variables, only Vjump, Vdonor, and Vgrid are readily adjustable in
the course of operation of a machine, the other variables being
substantially constant while the machine is running. Therefore, in
order to avoid arcing conditions, the values of Esolid and Ebkg
must be constrained so as not to exceed arcing conditions, and the
only practical way to constrain these values is to monitor and
control at least one of Vdonor, Vjump, and Vgrid while the machine
is in operation.
Another important parameter affecting whether arcing conditions
exist in a particular situation is the ambient air pressure, which
in turn generally relates to the elevation of a particular machine
relative to sea level. Once again, in general, the higher the
elevation of a particular machine, the higher the likelihood of
arcing conditions. Thus, according to one aspect of the present
invention, a key input parameter to a control system is a number
symbolic of the elevation of the particular machine. There are many
possible ways in which this number can be entered into a control
system. One option is to include a barometer or altimeter as part
of the machine itself, but this would add expense. It is simpler to
have service personnel enter the number relating to the elevation
when the machine is installed. The nature of this number can depend
on the sophistication of the system. The service personnel could
enter the more or less precise elevation of the installation site,
or more simply could just enter, via a control panel, a yes-or-no
indication that the elevation is above a certain threshold level,
such as over 4000 feet.
FIG. 4 is a flowchart illustrating the arcing-control aspect of a
control system for a xerographic printer according to the present
invention. It should be understood that what is shown in the Figure
is only a part of a general control method for maintaining print
quality. As such, the arcing-avoidance steps shown in the Figure
can be considered as "riding on" the more general control system
(not shown) by which overall desired print quality is achieved. A
control system with the single desired state of optimal print
quality, such as determined by readings from a densitometer
monitoring the developed images on photoreceptor 10, will at
various times require that different elements, such as donor roll
42 or corotron 22, have particular biases. In the course of
operation of the general control system, certain biases on various
elements may be demanded for the sake of print quality, and these
new biases may accidentally result in arcing conditions in the
development gap G. It is the general function of the present
invention, and in particular the steps shown in the Figure, to
detect conditions in which arcing is likely to occur, and then
alter the function of the general control system to avoid these
arcing conditions.
With particular reference to FIG. 4, at some initial time, such as
at installation of the machine at a site, an altitude is entered
into the system, such as shown at step 200. Once again, this
altitude may be determined by an instrument associated with the
machine, or entered by service personnel. The next step, shown as
202, is to convert this altitude to an associated arcing potential.
In other words, there is a known empirical relationship between the
elevation and the Paschen breakdown voltage. This empirical
relationship can be summarized, either precisely or roughly, by a
look up table which can readily be incorporated into the machine
itself. In one practical embodiment of the present invention, the
function describing this empirical relationship is set at a
constant 155 volts/mil gap width for any altitude from sea level to
4,000 ft., with a function sloping linearly from 155 volts/mil at
4,000 ft. to 120 volts/mil at 10,000 ft. In this way, arcing
conditions for a particular altitude can be looked up. It is a
matter of design choice, how close to the calculated breakdown
voltage the potential in a gap G will be allowed to approach. For
instance, if the breakdown voltage is determined to be 155
volts/mil, a risk-averse system could be contemplated which would
trigger a warning at 100 volts/mil, while in some situations 145
volts/mil would be considered acceptably far from arcing
conditions. Various threshold determination arrangements will be
apparent.
Once the altitude-dependent arcing conditions are determined, the
field strength of the development gap G is monitored while the
printing machine is running, which also means while the general
control system for optimizing print quality is running. According
to the present invention, on a reasonably regular basis, such as at
the start of every new job, or after an interval of a predetermined
number of prints, the values of Vjump and Vdonor which are at the
moment being demanded by the control system (step 204) are entered
into the equations described above, to determine a running value of
the field strength in the gap for both solid and background areas,
Esolid and Ebkg (step 206). At step 208, these running
determinations of Esolid and Ebkg are compared to the altitude
dependent breakdown voltage to determine whether arcing conditions
are being dangerously approached (step 210). If arcing conditions
are not being approached, the system simply waits for the next
interval, such as the next job over the next count of a certain
number of prints, to monitor Vjump and Vdonor yet again (step
212).
If, however, the current values of either Esolid and Ebkg approach
a predetermined threshold level near the breakdown voltage in which
arcing conditions would result, the system shown in FIG. 4 is
called upon to override the general control system to avoid this
dangerous condition, in particular by causing the control system to
constrain, either by an upper or lower bound, at least one of the
parameters which can be used to control the potential in
development gap G. In the particular embodiment, either Vjump,
Vdonor, or Vgrid can be constrained (step 214). Of course, it is
highly dependent on the overall nature of the control system for
obtaining optimal print quality which of these parameters is most
easily constrained to avoid arcing conditions while still
maintaining desirable print quality. If it is apparent that print
quality will suffer regardless of which parameter is constrained,
it may be desirable to provide a system in which the printing
apparatus is stopped and an error message is communicated to the
user, such as to the user interface shown as 120 in FIG. 1 and/or
over the internet (such as to service personnel). Alternately, in a
design of a xerographic development system in which a secondary
supply of toner-rich developer can be dispensed into the
development unit automatically (such as, in the embodiment of FIG.
1, dispensing developer or pure toner from dispenser 39 into
developer housing 40), it is possible to initiate a dispense of new
developer as a way of bringing the various biases into acceptable
ranges.
* * * * *